Oscillators are widely used in analog and digital circuits. They can be employed in a large number of applications, for example to drive a voltage multiplier, generate a clock frequency or create a programmable delay. Usually a well-designed oscillator should be able to operate at high switching frequency, have an oscillation frequency independent of the supply voltage and variations in process and temperature, have a duty cycle definable in accordance with a constant ratio and have low electromagnetic interference (EMI), i.e., primarily voltage edges with controlled and not too high slope.
A very simple oscillating circuit can be achieved by connecting in a loop two inverters, one resistor and one capacitor and generating an oscillating electric signal having an essentially square shape. Naturally this simple circuit does not meet the above mentioned requisites and in particular the oscillation frequency depends both on the supply voltage and the switching threshold of the inverters and hence on the temperature and the process.
From the article by S. Hobrecht, "An Intelligent BiCMOS/DMOS Quad 1-A High-Side Switch", IEEE Journal of Solid-State Circuits, Vol. 25, No. 6, December 1990, and incorporated herein by reference, and in particular FIG. 4 and the corresponding description on page 1397, a more sophisticated circuit is known and shown in the annexed FIG. 1.
It comprises a first inverter INV1, a second inverter INV2 and two symmetrical and analogous sections, the first of which comprises a first capacitor C1 having a first and a second terminals, a first current generator GEN1 having an input terminal and an output terminal, a first transistor SW1 of the p-channel MOS type and a first controlled switch SW3. Analogously the second section comprises a second capacitor C2 having a first and a second terminal, a second current generator GEN2 having an input terminal and an output terminal, a second transistor SW2 of n-channel MOS type and a second controlled switch SW4.
The circuit is powered by means of connection to a ground terminal GND and a power supply terminal VS. The output terminal OUT is connected to the output of the inverter INV2.
If the two capacitors have the same capacitance Co and the generators GEN1 and GEN2 have the same current Io, oscillation frequency will be: ##EQU1## and hence depends on the supply voltage; the duty cycle depends on the switching threshold of the inverters (and hence on the temperature and process); and in addition, despite the fact that the voltages across the capacitors have a triangular shape which should involve a low electromagnetic interference, this interference is high enough that the falling edges have a rather high slope (the shape is more correctly termed "saw tooth") as shown in FIG. 4 of this article.
Of course this circuit is very sensitive to the symmetry of its components.
The purpose of the present invention is to supply a method of generating an oscillating electric signal and an associated oscillating circuit overcoming the shortcomings of the prior art.